Patent application title: OPTICAL GLASS

Abstract:

An optical glass of the present invention has optical constants of a
refractive index (nd) within a range from 1.75 to 1.85 and an Abbe number
(ν d) within a range from 35 to 45, comprises SiO2 and
B2O3 as essential components and one or more components
selected from the group consisting of ZrO2, Nb2O5,
Ta2O5 and WO3, has a glass transition temperature (Tg) of
580° C. or below and has weathering resistance (surface method) of
Class 1 or Class 2.

Claims:

1. An optical glass having optical constants of a refractive index (nd)
within a range from 1.75 to 1.85 and an Abbe number (ν d) within a
range from 35 to 45, comprising SiO2 and B2O3 as essential
components and one or more components selected from the group consisting
of ZrO2, Nb2O5, Ta2O5 and WO3, having a
glass transition temperature (Tg) of 580.degree. C. or below and having
weathering resistance (surface method) of Class 1 or Class 2.

2. An optical glass having optical constants of a refractive index (nd)
within a range from 1.75 to 1.85 and an Abbe number (ν d) within a
range from 35 to 45, comprising SiO2, B2O3,
La2O3, ZrO2, Nb2O5, Ta2O5, WO3,
ZnO and Li2O as essential components, being substantially free of
lead component, arsenic component and fluorine component, having a total
amount of said oxides exceeding 96 mass %, having a ratio of
ZnO/SiO2 of 2.8 or over, having a glass transition temperature (Tg)
of 580.degree. C. or below and having weathering resistance (surface
method) of Class 1 or Class 2.

3. An optical glass as defined in claim 1 wherein a ratio of
B2O3/SiO2 is 3.2 or below.

4. An optical glass as defined in claim 1 having a ratio of ZnO/SiO2
within a range from 3.3 to 3.7 and having the weathering resistance
(surface method) of Class 1.

where RO is one or more components selected from the group consisting of
MgO, CaO, SrO and BaO, and/or
TABLE-US-00032
Sb2O3 0-1%.

17. A lens perform made of an optical glass as defined in claim 2.

18. An optical element formed of a lens perform as defined in claim 17.

19. An optical element formed of an optical glass as defined in claim 2 by
precision press molding.

Description:

TECHNICAL FIELD

[0001]This invention relates to an optical glass and, more particularly,
to an optical glass having a low glass transition temperature (Tg) and
high refractive index and low dispersion characteristics, and excellent
chemical durability, particularly weathering resistance (surface method),
and being suitable for precision press molding.

PRIOR ART

[0002]There are spherical lenses and aspherical lenses as lenses used for
constituting an optical system. Many spherical lenses are produced by
lapping and polishing glass pressings obtained by reheat press molding
glass materials. On the other hand, aspherical lenses are mainly produced
by precision press molding, i.e., the method according to which lens
preforms which have been softened by heating are press molded with a mold
having a high precision molding surface and the shape of the high
precision molding surface of the mold is transferred to the lens
preforms.

[0003]In obtaining glass moldings such as aspherical lenses by precision
press molding, it is necessary to press lens preforms which have been
softened by heating in a high temperature environment for transferring
the shape of the high precision molding surface of the mold to the lens
preforms and, therefore, the mold used for such precision press molding
is subjected to a high temperature and, moreover, a high pressing force
is applied to the mold. Hence, in heating and softening the lens preforms
and press molding the lens preforms, the molding surface of the mold
tends to be oxidized or eroded, or a release film provided on the molding
surface tends to be damaged with the result that the high precision
molding surface of the mold cannot be maintained or the mold itself tends
to be damaged. In such a case, the mold must be replaced and, as a
result, frequency of replacement of the mold increases and production of
products at a low cost in a large scale thereby cannot be achieved.
Accordingly, glasses used for precision press molding are desired to have
the lowest possible glass transition temperature (Tg) from the standpoint
of preventing such damage to the mold, maintaining the high precision
molding surface of the mold for a long period of time and enabling
precision press molding at a low pressing force.

[0004]In conducting precision press molding, the glass of a lens preform
needs to have a mirror surface or a surface close to a mirror surface. A
lens preform generally is either produced directly from molten glass by
the dripping method or produced by lapping and polishing glass pieces.
The dripping method is more generally employed in view of advantages in
the cost and number of processing steps. The lens preform produced by the
dripping method is called gob or glass gob.

[0005]Optical glasses used for precision press molding, however, have
drawbacks that their chemical durability is generally so poor that fading
is observed on the surface of the lens preforms made of these optical
glasses with resulting difficulty in maintaining a mirror surface or a
surface which is close to a mirror surface. This particularly poses a
problem in storing gob after it is produced and the surface method
weathering resistance in chemical durability is an important property
required for optical glasses used for precision press molding.

[0006]For these reasons, from the point of view of utility for optical
design, there has been a strong demand for an optical glass having high
refractive index and low dispersion characteristics, a low glass
transition temperature (Tg) and excellent weathering resistance (surface
method).

[0007]There has particularly been a strong demand for a high refractive
index and low dispersion optical glass having optical constants of
refractive index (nd) within a range from 1.75 to 1.85 and Abbe number
(ν d) within a range from 35 to 45.

[0008]Since a high refractive index and low dispersion optical glass is
very useful in the optical design, various glasses of this type have for
a long time been proposed.

[0009]Japanese Patent Application Laid-open Publications No. Hei 6-305769,
No. 2002-362938, No. 2003-201142, No. 2002-12443 and No. 2003-252647
disclose optical glasses having a low glass transition temperature (Tg).
It has been found, however, that optical glasses which are specifically
disclosed in these publications are insufficient in the weathering
resistance (surface method), for reasons that a ratio of ZnO/SiO2 in
mass % is outside of the range of 2.8 or over, a ratio of
B2O3/SiO2 is outside of the range of 3.2 or below, and a
total amount of SiO2, B2O3, La2O3, ZrO2,
Nb2O5, Ta2O5, WO3, ZnO and Li2O is outside
of the range exceeding 96 mass %.

[0010]It is, therefore, an object of the present invention to provide an
optical glass which has comprehensively eliminated the above described
drawbacks of the prior art optical glasses and has the above described
optical constants, a low glass transition temperature (Tg) and an
excellent weathering resistance (surface method) and therefore is
suitable for precision press molding.

DISCLOSURE OF THE INVENTION

[0011]Studies and experiments made by the inventor of the present
invention for achieving the above described object of the invention have
resulted in the finding, which has led to the present invention, that by
adopting a composition comprising specific amounts of SiO2,
B2O3, La2O3, ZrO2, Nb2O3,
Ta2O5, WO3, ZnO and La2O3 an optical glass
having the above described optical constants, a low glass transition
temperature (Tg), excellent weathering resistance (surface method), and
being suitable for precision press molding can be obtained.

[0012]For achieving the above described object of the invention, in the
first aspect of the invention, there is provided an optical glass having
optical constants of a refractive index (nd) within a range from 1.75 to
1.85 and an Abbe number (ν d) within a range from 35 to 45, comprising
SiO2 and B2O3 as essential components and one or more
components selected from the group consisting of ZrO2,
Nb2O5, Ta2O5 and WO3, having a glass transition
temperature (Tg) of 580° C. or below and having weathering
resistance (surface method) of Class 1 or Class 2.

[0013]In the second aspect of the invention, there is provided an optical
glass having optical constants of a refractive index (nd) within a range
from 1.75 to 1.85 and an Abbe number (ν d) within a range from 35 to
45, comprising SiO2, B2O3, La2O3, ZrO2,
Nb2O5, Ta2O5, WO3, ZnO and Li2O as
essential components, being substantially free of lead component, arsenic
component and fluorine component, having a total amount of said oxides
exceeding 96 mass %, having a ratio of ZnO/SiO2 of 2.8 or over,
having a glass transition temperature (Tg) of 580° C. or below and
having weathering resistance (surface method) of Class 1 or Class 2.

[0014]In the third aspect of the invention, there is provided an optical
glass as defined in the first or second aspect wherein a ratio of
B2O3/SiO2 is 3.2 or below.

[0015]In the fourth aspect of the invention, there is provided an optical
glass as defined in any of the first to third aspects having a ratio of 1
ZnO/SiO2 within a range from 3.3 to 3.7 and having the weathering
resistance (surface method) of Class 1.

[0016]In the fifth aspect of the invention, there is provided an optical
glass as defined in any of the first to fourth aspects comprising, in
mass %:

[0022]where RO is one or more components selected from the group
consisting of MgO, CaO, SrO and BaO, and/or

TABLE-US-00007
Sb2O3 0-1%.

[0023]In the ninth aspect of the invention, there is provided a lens
perform made of an optical glass as defined in any of the first to eighth
aspects.

[0024]In the tenth aspect of the invention, there is provided an optical
element formed of a lens perform as defined in the ninth aspect.

[0025]In the eleventh aspect of the invention, there is provided an
optical element formed of an optical glass as defined in any of the first
to eighth aspects by precision press molding.

[0026]According to the invention, there is provided an optical glass which
has the above described optical constants, a low glass transition
temperature (Tg) and an excellent weathering resistance (surface method)
and therefore is suitable for precision press molding.

BEST MODE FOR CARRYING OUT THE INVENTION

[0027]Description will be made about components which the optical glass of
the present invention can comprise. Unless otherwise described, the
composition ratio of each component will be expressed in mass %.

[0028]SiO2 is an indispensable component which is very effective for
increasing viscosity of the glass and improving resistance to
devitrification and weathering resistance (surface method) of the glass.
If, however, the amount of this component is insufficient, these effects
cannot be achieved sufficiently whereas if the amount of this component
is excessively large, the glass transition temperature (Tg) rises and the
melting property of the glass is deteriorated. Therefore, the lower limit
of the amount of this component should preferably be 6%, more preferably
6.01% and, most preferably be 6.02% and the upper limit of the amount of
this component should be 10%, more preferably 9% and, most preferably be
8%. SiO2 can be incorporated in the glass by using, e.g., SiO2
as a raw material.

[0029]In the optical glass of the present invention which is a lanthanum
glass, B2O3 is an indispensable component as a glass forming
oxide. If, however, the amount of this component is insufficient,
resistance to devitrification becomes insufficient whereas if the amount
of this component excessively large, weathering resistance (surface
method) is deteriorated. Therefore, the lower limit of the amount of this
component should preferably be 12%, more preferably 13% and, most
preferably be 14% and the upper limit of this component should preferably
be 24%, more preferably 22% and, most preferably be 20%. B2O3
can be incorporated in the glass by using, e.g., H3BO3 or
B2O3 as a raw material. . . .

[0030]La2O3 is an indispensable component which is effective for
increasing refractive index and lowering dispersion. If, however, the
amount of this component is insufficient, it is difficult to maintain the
optical constants within the above described values whereas if the amount
of this component is excessively large, resistance to devitrification is
deteriorated. Therefore, the lower limit of the amount of this component
should preferably be 26%, more preferably 28% and, most preferably be 30%
and the upper limit of the amount of this component should preferably be
42%, more preferably 41% and, most preferably be 40%. La2O3 can
be incorporated in the glass by using, e.g., La2O3, lanthanum
nitrate or its hydrate as a raw material.

[0031]ZrO2 is an indispensable component which is very effective for
adjusting optical constants, improving resistance to devitrification and
improving weathering resistance (surface method). If, however, the amount
of this component is insufficient, these effects cannot be achieved
sufficiently whereas if the amount of this component is excessively
large, resistance to devitrification is deteriorated and it becomes
difficult to maintain the glass transition temperature (Tg) at a desired
low temperature. Therefore, lower limit of the amount of this component
should preferably be 1.5%, more preferably 1.55% and, most preferably be
1.6% and the upper limit of the amount of this component should
preferably be 10%, more preferably 8% and, most preferably be 6%.
ZrO2 can be incorporated in the glass by using, e.g., ZrO2 as a
raw material.

[0032]Nb2O5 is effective for increasing refractive index and
improving weathering resistance (surface method) and resistance to
devitrification. If, however, the amount of this component is
insufficient, these effects cannot be sufficiently achieved whereas if
the amount of this component is excessively large, resistance to
devitrification decreases rather than increases. Therefore, the lower
limit of this component should preferably be 1%, more preferably 2% and,
most preferably be 3% and the upper limit of the amount of this component
should preferably be 10%, more preferably 9% and, most preferably, be 8%.
Nb2O5 can be incorporated in the glass by using, e.g.,
Nb2O5 as a raw material.

[0033]Ta2O5 is an indispensable component which is very
effective for increasing refractive index and, improving weathering
resistance (surface method) and resistance to devitrification. If,
however, the amount of this component is insufficient, these effects
cannot be achieved sufficiently whereas if the amount of this component
is excessively large, it becomes difficult to maintain the above
described optical constants. Therefore, lower limit of the amount of this
component should preferably be 1%, more preferably 2% and, most
preferably be 3% and the upper limit of the amount of this component
should preferably be 15%, more preferably 13% and, most preferably be
10%. Ta2O5 can be incorporated in the glass by using, e.g.,
Ta2O5 as a raw material.

[0034]WO3 is effective for adjusting optical constants and improving
resistance to devitrification. If, however, the amount of this component
is insufficient, these effects cannot be achieved sufficiently whereas if
the amount of this component is excessively large, resistance to
devitrification and transmittance in the short wavelength region of the
visible ray region are deteriorated. Therefore, the lower limit of this
component should preferably be 1%, more preferably 2% and, most
preferably, be 3% and the upper limit of the amount of this component
should preferably be 10%, more preferably 9% and, most preferably, be 8%.
WO3 can be incorporated in the glass by using, e.g., WO3 as a
raw material.

[0035]ZnO is an indispensable component which is effective for lowering
the glass transition temperature (Tg). If, however, the amount of this
component is insufficient, this effect cannot be achieved sufficiently
whereas if the amount of this component is excessively large, resistance
to devitrification is deteriorated. Therefore, lower limit of the amount
of this component should preferably be 16%, more preferably more than 20%
and, most preferably be 20.5% and the upper limit of the amount of this
component should preferably be 26%, more preferably 25% and, most
preferably be 24%. ZnO can be incorporated in the glass by using, e.g.,
ZnO as a raw material.

[0036]Li2O is an indispensable component which is effective for
lowering the glass transition temperature (Tg) substantially and
facilitating melting of mixed glass materials. If, however, the amount of
this component is insufficient, these effects cannot be achieved
sufficiently whereas if the amount of this component is excessively
large, resistance to devitrification is sharply deteriorated. Therefore,
the lower limit of the amount of this component should preferably be
0.6%, more preferably 0.8% and, most preferably be 1% and the upper limit
of the amount of this component should preferably be 4%, more preferably
3.5% and, most preferably be 3%. Li2O can be incorporated in the
glass by using, e.g., Li2O, Li2CO3, LiOH or LiNO3 as
a raw material.

[0037]Sb2O3 may be optionally added for defoaming during melting
of the glass. If the amount of this component is excessive, transmittance
in the short-wave region of the visible ray region is deteriorated.
Therefore, the upper limit of the amount of this component should
preferably be 1%, more preferably 0.8% and, most preferably, be 0.5%.

[0038]Gd2O3 is effective for increasing refractive index and
lowering dispersion. If, however, the amount of this component is
excessively large, resistance to devitrification and weathering
resistance (surface method) are deteriorated. Therefore, the upper limit
of the amount of this component should preferably be 10%, more preferably
less than 4% and, most preferably be 2%. Gd2O3 can be
incorporated in the glass by using, e.g., Gd2O3 as a raw
material.

[0039]GeO2 is effective for increasing refractive index and improving
resistance to devitrification. Since, however, this component is very
expensive, the upper limit of the amount of this component should
preferably be 5%, more preferably less than 4% and, most preferably, be
2%. GeO2 can be incorporated in the glass by using, e.g., GeO2
as a raw material.

[0040]Al2O3 is effective for improving chemical durability and
particularly weathering resistance (surface method). If the amount of
this component is excessively large, resistance to devitrification is
deteriorated. Therefore, the upper limit of this component should
preferably be 10%, more preferably less than 4% and, most preferably, be
2%. Al2O3 can be incorporated in the glass by using, e.g.,
Al2O3 or Al(OH)3.

[0041]TiO2 is effective for adjusting optical constants and improving
resistance to devitrification. If, however, the amount of this component
is excessively large, resistance to devitrification decreases rather than
increases. Therefore the upper limit of the amount of this component
should preferably be 10%, more preferably less than 4% and, most
preferably be 2%. TiO2 can be incorporated in the glass by using,
e.g., TiO2 as a raw material.

[0042]RO (one or more components selected from the group consisting of
MgO, CaO, SrO and BaO) is effective for adjusting optical constants. If,
however, the amount of this component is excessively large, resistance to
devitrification is deteriorated. Therefore, the upper limit of the amount
of this component should preferably be 10%, more preferably less than 4%
and, most preferably be 2%. RO can be incorporated in the glass by using,
e.g., MgO, CaO, SrO or BaO, or its carbonate, nitrate or hydroxide as a
raw material.

[0043]The above described raw materials used in the respective components
of the glass have been cited for illustrative purpose only and raw
materials which can be used for the glass of the present invention are
not limited to the above described oxides etc. but can be selected from
known materials in accordance with various modifications of manufacturing
conditions for manufacturing the glass.

[0044]The inventor of the present invention has found that, by adjusting
the ratio of amounts of B2O3 to SiO2 to a predetermined
range, weathering resistance (surface method) of the glass is improved.
More specifically, the ratio of B2O3/SiO2 should
preferably be 3.2 or below, more preferably 3.18 or below and, most
preferably be 3.17 or below.

[0045]The inventor of the present invention has found that in an optical
glass having optical constants within the above described ranges, the
weathering resistance (surface method) can be improved by adjusting the
ratio of ZnO to SiO2 to a predetermined value. More specifically,
the ratio of ZnO/SiO2 should preferably be 2.8 or over, and more
preferably be 2.82 or over. For achieving the weathering resistance
(surface method) of Class 1, the ratio of ZnO/SiO2 should preferably
be within a range from 3.3 to 3.7.

[0046]The inventor of the present invention has also found that in an
optical glass having optical constants within the above described ranges,
the weathering resistance (surface method) can be improved when the total
amount of SiO2, B2O3, La2O3, ZrO2,
Nb2O5, Ta2O5, WO3, ZnO and Li2O exceeds 96
mass %. Therefore, this total amount should preferably exceed 96 mass %,
more preferably 97 mass % or over and, most preferably, be 99 mass % or
over.

[0047]Further, for maintaining desired optical constants and maintaining
excellent weathering resistance, the total amount of SiO2,
B2O3, La2O3, ZrO2, Nb2O5.
Ta2O5, WO3, ZnO and Li2O, the ratio of
B2O3/SiO2 and the ratio of ZnO/SiO2 should preferably
be within the above described desirable ranges simultaneously.

[0048]In the present specification, "weathering resistance (surface
method)" which is a property defined on the assumption that a lens
preform, i.e., gob, is stored for a certain period of time before
precision press molding indicates degree of fading occurring when a lens
preform, i.e., gob, is exposed for a predetermined period of time in an
environment in which it is stored

[0049]More specifically, for measuring weathering resistance (surface
method), a test piece having a polished surface of 30 mm×30
mm×3 mm is exposed in a constant temperature and constant humidity
bath of 50° C., 85% relative humidity for 24 hours and then the
polished surface is inspected by a microscope of 50 magnifications for
observing state of fading of the polished surface. According to the
standard of evaluation by this method, Class 1 indicates that no fading
is observed when a test piece which has been tested for 24 hours is
inspected at 6,000 luxes, Class 2 indicates that fading is not observed
at 1,500 luxes but is observed at 6,000 luxes, Class 3 indicates that
fading is observed at 1,500 luxes and Class 4 indicates that, when a test
piece of Class 3 is further exposed in a constant temperature and
constant humidity bath of 50° C., 85% relative humidity for 6
hours and then its polished surface is inspected by a microscope of 50
magnifications, fading is observed at 1,500 luxes. If no fading is
observed in this further tested test piece, the test piece remains to be
Class 3. This measuring method is a known method described in OHARA
OPTICAL GLASS Catalog 2002J, page 13.

[0051]The glass may comprise Lu2O3, Hf2O3, SnO2,
Ga2O3, Bi2O3 and BeO. Since Lu2O3,
Hf2O3 and Ga2O3 are expensive materials, use of these
components increases the manufacturing cost and it is not practical to
use these components in commercial production. As to SnO2, there is
likelihood that, when glass materials are melted in a platinum crucible
or a melting furnace which is formed with platinum in a portion which
comes into contact with molten glass, tin of SnO2 is alloyed with
platinum and heat resisting property of the alloyed portion is
deteriorated with resulting making of a hole in the alloyed portion and
leakage of the molten glass from the hole. Bi2O3 and BeO have
the problem that these components adversely affect the environment and
therefore impose a heavy burden to the environment. Accordingly, the
upper limit of the amount of each of these components should preferably
be less than 0.1%, more preferably 0.05% and, most preferably these
components should not be added at all.

[0052]Y2O3 may be added as n optional component but this
componnent has a problem of deteriorating resistance to devitrification
significantly. Therefore, the upper limit of this component should
preferably be less than 0.1%, more preferably 0.05% and, most preferably,
should not be added at all.

[0053]Description will now be made about components which the optical
glass of the present invention should not comprise.

[0054]Fluorine causes occurrence of striae in the production of a gob for
a lens preform and therefore makes it difficult to produce a gob.
Fluorine therefore should not be added to the optical glass of the
present invention.

[0055]A lead compound not only has the problem that it tends to be fused
with the mold during precision press molding, has the problem that steps
must be taken for protecting the environment not only in production of
the glass but also in cold processing such as polishing and waste of the
glass and therefore it imposes a heavy burden to the environment. The
lead compound therefore should not be added to the optical glass of the
present invention.

[0056]As2O3, cadmium and thorium adversely affect the
environment and therefore impose a heavy burden to the environment. These
components therefore should not be added to the optical glass of the
present invention.

[0057]P2O5 tends to deteriorate resistance to devitrification
when it is added to the glass and, therefore, it is not preferable to add
P2O5 to the optical glass of the present invention.

[0058]As to TeO2, there is likelihood that, when glass materials are
melted in a platinum crucible or a melting furnace which is formed with
platinum in a portion which comes into contact with molten glass, tin of
TeO2 is alloyed with platinum and heat resisting property of the
alloyed portion is deteriorated with resulting making of a hole in the
alloyed portion and leakage of the molten glass from the hole. TeO2
therefore should not be added to the optical glass of the present
invention.

[0059]The optical glass of the present invention should preferably not
comprise coloring components such as V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Eu,
Nd, Sm, Tb, Dy and Er. That is to say, these coloring components should
not be intentionally added except for a case where these components are
mixed as impurities.

[0060]Since the glass composition of the present invention is express in
mass %, it cannot be directly expressed in mol %. A composition expressed
in mol % of respective oxides existing in the glass composition
satisfying the properties required by the present invention generally
assumes the following values:

[0061]where RO is one or more components selected from the group
consisting of MgO, CaO, SrO and BaO, and/or

TABLE-US-00009
Sb2O3 0-0.5%.

[0062]In the optical glass of the present invention, SiO2 is an
indispensable component which is very effective for increasing viscosity
of the glass and improving resistance to devitrification and weathering
resistance (surface method) of the glass. The upper limit of the amount
of this component should preferably be 20 mol %, more preferably 17 mol %
and, most preferably be 14 mol % and the lower limit of the amount of
this component should be 10 mol %, more preferably 10.5 mol % and, most
preferably be 11 mol %.

[0063]In the optical glass of the present invention, B2O3 is an
indispensable component as a glass forming oxide and a component
improving resistance to devitrification. The upper limit of the amount of
this component should preferably be 45 mol %, more preferably 40 mol %
and, most preferably be 35 mol % and the lower limit of this component
should preferably be 20 mol %, more preferably 24 mol % and, most
preferably be 26 mol %. . . .

[0064]In the optical glass of the present invention, La2O3 is
effective for increasing refractive index and lowering dispersion. The
upper limit of the amount of this component should preferably be 17 mol
%, more preferably 16 mol % and, most preferably be 15 mol % and the
lower limit of the amount of this component should preferably be 7 mol %,
more preferably 8 mol % and, most preferably be 9 mol %.

[0065]In the optical glass of the present invention, ZrO2 is
effective for adjusting optical constants, improving resistance to
devitrification and improving weathering resistance (surface method). The
upper limit of the amount of this component should preferably be 10 mol
%, more preferably 8 mol % and, most preferably be 6 mol % and the lower
limit of the amount of this component should preferably be 1.5 mol %,
more preferably 1.55 mol % and, most preferably be 1.6 mol %.

[0066]In the optical glass of the present invention, Nb2O5 is
effective for increasing refractive index and improving weathering
resistance (surface method) and resistance to devitrification. The upper
limit of the amount of this component should preferably be 5 mol %, more
preferably 4.5 mol % and, most preferably, be 4 mol % and the lower limit
of the amount of this component should preferably be 0.5 mol %, more
preferably 1 mol % and, most preferably, be 1.5 mol %.

[0067]In the optical glass of the present invention, Ta2O5 is
effective for increasing refractive index and improving weathering
resistance (surface method) and resistance to devitrification. The upper
limit of the amount of this component should preferably be 3.5 mol %,
more preferably 3 mol % and, most preferably be 2.6 mol % and the lower
limit of the amount of this component should preferably be 0.2 mol %,
more preferably 0.5 mol % and, most preferably be 0.8 mol %.

[0068]In the optical glass of the present invention, WO3 is effective
for adjusting optical constants and improving resistance to
devitrification. The upper limit of the amount of this component should
preferably be 5 mol %, more preferably 4.5 mol % and, most preferably, be
4 mol % and the lower limit of the amount of this component should
preferably be 0.5 mol % more preferably 1 mol % and, most preferably, be
1.5 mol %.

[0069]In the optical glass of the present invention, ZnO is effective for
lowering the glass transition temperature (Tg). The upper limit of the
amount of this component should preferably be 38 mol %, more preferably
36 mol % and, most preferably be 34 mol % and the lower limit of the
amount of this component should preferably be 20 mol %, more preferably
22 mol % and, most preferably be 24 mol %.

[0070]In the optical glass of the present invention, Li2O is
effective for lowering the glass transition temperature (Tg)
substantially and facilitating melting of mixed glass materials. The
upper limit of the amount of this component should preferably be 16 mol
%, more preferably 14 mol % and, most preferably be 12 mol % and the
lower limit of the amount of this component should preferably be 2 mol %,
more preferably 3 mol % and, most preferably be 3.5 mol %.

[0071]In the optical glass of the present invention, Sb2O3 is
effective for defoaming during melting of the glass. The upper limit of
the amount of this component should preferably be 0.5 mol %, more
preferably 0.3 mol % and, most preferably, be 0.1 mol %.

[0072]In the optical glass of the present invention, Gd2O3 is
effective for increasing refractive index and lowering dispersion. The
upper limit of the amount of this component should preferably be less
than 4 mol %, more preferably 3 mol % and, most preferably be 1 mol %.

[0073]In the optical glass of the present invention, GeO2 is
effective for increasing refractive index and improving resistance to
devitrification. The upper limit of the amount of this component should
preferably be less than 4 mol % more preferably 3 mol % and, most
preferably, be 1 mol %.

[0074]In the optical glass of the present invention, Al2O3 is
effective for improving weathering resistance (surface method). The upper
limit of the amount of this component should preferably be less than 4
mol %, more preferably 3 mol % and, most preferably, be 1 mol %.

[0075]In the optical glass of the present invention, TiO2 is
effective for adjusting optical constants and improving resistance to
devitrification. The upper limit of the amount of this component should
preferably be less than 4 mol %, more preferably 3 mol % and, most
preferably be 1 mol %.

[0076]In the optical glass of the present invention, RO (one or more
components selected from the group consisting of MgO, CaO, SrO and BaO)
is effective for adjusting optical constants. The upper limit of the
amount of this component should preferably be less than 4 mol %, more
preferably 3 mol % and, most preferably be 1 mol %.

[0077]Description will now be made about the properties of the optical
glass of the present invention.

[0078]As described above, the optical glass of the present invention
should preferably have, from the standpoint of utility in the optical
design, optical constants of a refractive index (nd) within a range from
1.75 to 1.85 and an Abbe number (ν d) within a range from 35 to 45,
more preferably a refractive index (nd) within a range from 1.76 to less
than 1.84 and an Abbe number (ν d) within a range from 35 to 45 and,
most preferably, a refractive index (nd) within a range from 1.76 to 1.84
and an Abbe number (ν d) within a range from 36 to 44.

[0079]In the optical glass of the present invention, an excessively high
Tg tends to cause, as described previously, deterioration in the mold in
conducting precision press molding. In the optical glass of the present
invention, therefore, the upper limit of Tg should preferably be
580° C., more preferably 550° C. and, most preferably, be
530° C.

[0080]Yield point At should preferably be 620° C., more preferably
610° C. and, most preferably be 580° C. or below.

[0081]In the optical glass of the present invention, for realizing a
stable production by the manufacturing method to be described later, it
is important to maintain liquidus temperature of the glass below
1080° C. or below. A preferable liquidus temperature is
1050° C. or below and particularly preferable liquidus temperature
is 1020° C. or below because, at this liquidus temperature, the
range of viscosity which enables a stable production is broadened and the
melting temperature of the glass is lowered and energy consumption
thereby can be reduced.

[0082]The liquidus temperature means the lowest temperature at which no
crystal is observed when crushed glass specimen is put on a platinum
plate, held in a furnace with temperature graduations for 30 minutes and
thereafter is taken out of the furnace and, after cooling, presence or
absence of crystals in the softened glass is observed with a microscope.

[0083]As described previously, the optical glass of the present invention
can be used as a preform for press molding or, alternatively, molten
glass can be directly pressed. In a case where it is used as a preform,
the method for manufacturing the preform and the manner of precision
press molding are not particularly limited but known manufacturing method
and known precision press molding method can be used. As a method for
manufacturing a preform, a preform can be made in a manner as described
in the gob forming method disclosed in Japanese Patent Application
Laid-open Publication No. Hei 8-319124 or a preform can be made directly
from molten glass as described in the manufacturing method and apparatus
of an optical glass disclosed in Japanese Patent Application Laid-open
Publication No. Hei 8-73229. A preform can also be made by cold
processing a strip material.

[0084]In a case where a preform is made by dripping molten glass by using
the optical glass of the present invention, if viscosity of the molten
glass is too low, striae tends to occur in the preform whereas if
viscosity is too high, cutting of glass by weight and surface tension of
dripping glass becomes difficult.

[0085]Accordingly, for producing a high-quality preform stably, logarithm
log η of viscosity (Pas) should preferably be within a range from 0.4
to 2.0, more preferably within a range from 0.5 to 1.8 and, most
preferably be within a range from 0.6 to 1.6.

[0086]Although the method of precision press molding a preform is not
limited, a method as disclosed in Japanese Patent Application Laid-open
Publication No. Sho 62-41180, Method for Forming an Optical Element, may
for example be used.

EXAMPLES

[0087]Examples of the present invention will now be described

[0088]though the present invention in no way is limited by these examples.

[0089]Tables 1 to 10 show compositions of Example No. 1 to No. 47 of the
optical glass of the present invention together with their refractive
index (nd), Abbe number (ν d), glass transition temperature (Tg),
yield point (At) and weathering resistance (surface method). In the
tables, composition of the respective components are expressed in mass %.

[0091]For manufacturing the glasses of Example No. 1 to No. 47 shown in
Tables 1 to 10, ordinary raw materials for an optical glass including
oxides, carbonates and nitrates were weighed and mixed so as to realize
the composition ratio of the respective examples shown in Tables 1 to 10.
The raw materials were put in a platinum crucible and melted at a
temperature within a range from 1000° C. to 1300° C. for
three to five hours depending upon the melting property of the
composition. After refining and stirring the melt for homogenization, the
melt was cast into a mold and annealed to provide the glasses.

[0092]Refractive index (nd) and Abbe number (ν d) of the glasses were
measured with respect to glasses which were obtained by setting the rate
of lowering of annealing temperature at -25° C./Hr.

[0093]Glass transition temperature (Tg) of the glasses was measured in
accordance with the Japan Optical Glass Industry Standard
JOGIS08-2003 "Measuring Method of Thermal Expansion of Optical
Glass". A specimen having length of 50 mm and diameter of 4 mm was used
as a test specimen.

[0094]Yield point (At) was measured in the same manner as in measuring
glass transition temperature (Tg), and a temperature at which stretching
of the glass ceased and shrinking of the glass started was adopted as
yield point.

[0095]Weathering resistance (surface method) was determined by conducting
the following test.

[0096]A test piece having a polished surface of 30 mm×30 mm×3
mm was exposed in a constant temperature and constant humidity bath of
50° C., 85% relative humidity for 24 hours and then the polished
surface was inspected by a microscope of 50 magnifications for observing
state of fading of the polished surface. In this method, Class 1
indicates that no fading is observed when a test piece which has been
tested for 24 hours is inspected at 6,000 luxes, Class 2 indicates that
fading is not observed at 1,500 luxes but is observed at 6,000 luxes,
Class 3 indicates that fading is observed at 1,500 luxes and Class 4
indicates that, when a test piece of Class 3 is further exposed in a
constant temperature and constant humidity bath of 50° C., 85%
relative humidity for 6 hours and then its polished surface is inspected
by a microscope of 50 magnifications, fading is observed at 1,500 luxes.
If no fading is observed in this further tested test piece, the test
piece remains to be Class 3.

[0097]As shown in Tables 1 to 10, the optical glasses of Example No. 1 to
No. 47 all have the optical constants (refractive index (nd) and Abbe
number (ν d) of the above described ranges and their glass transition
temperature (Tg) is 580° C. or below and, therefore, they are
suitable for precision press molding. They have also excellent weathering
resistance (surface method) and, therefore, have excellent chemical
durability.

[0098]On the other hand, the specimens of Comparative Example A to C shown
in Table 11 were manufactured under the same conditions as the examples
of the present invention were manufactured and the manufactured glasses
were evaluated by the same evaluation methods as used for evaluating the
examples of the present invention. In Comparative Example A to C, the
ratio of B2O3/SiO2 exceeded 3.2 and, therefore, they have
poor weathering resistance (surface method).

INDUSTRIAL APPLICABILITY

[0099]As described above, the optical glass of the present invention which
is of a SiO2-- B2O3--
La2O3--ZrO2--Nb2O5--
Ta2O5--WO3--ZnO--Li2O glass is free of Pb, As and F
and has optical constants of a refractive index (nd) within a range from
1.75 to 1.85 and an Abbe number (ν d) within a range from 35 to 45 and
glass transition temperature (Tg) of 580° C. or below and hence is
suitable for precision press molding and has sufficient industrial
utility.

[0100]Moreover, since the optical glass of the present invention has
excellent weathering resistance (surface method), in a case where a lens
preform, i.e., gob, is stored before precision press molding, there is no
likelihood of occurrence of fading due to exposure of the gob for a
certain period of time during storage and, therefore, the optical glass
can be very easily treated.